1. Field of the Invention
The present invention relates to a multiple tuning circuit for use in a nuclear magnetic resonance (NMR) spectrometer and, more particularly, to such a multiple tuning circuit having improved resistance to RF voltages.
2. Description of Related Art
FIG. 1 shows a conventional multiple tuning circuit designed to irradiate two species of low-frequency resonating nuclei having resonant frequencies of LF1 and LF2, respectively, with RF signals simultaneously and to detect resulting NMR signals. The RF frequency LF1 is higher than the LF2. For example, LF1 corresponds to the resonant frequency of 13C nucleus, while LF2 corresponds to the resonant frequency of 17O nucleus.
In FIG. 1, a sample coil 1 consists of a solenoid coil or saddle coil. Inductors 2 and 3 are directly connected to the opposite ends of the sample coil 1. One end of the inductor 2 is connected with the sample coil 1, the other end being grounded via a capacitor 6 or directly. Similarly, one end of the inductor 3 is connected with the sample coil 1, the other end being grounded via a capacitor 9 or directly. In the illustrated example, the two inductors are grounded indirectly via capacitors.
The opposite ends of the sample coil 1 are grounded via two tuning capacitors 4 and 7, respectively, for LF1. A tuning variable capacitor 10 for LF1 is connected with one end of the sample coil 1 in a parallel relation to the tuning capacitor 7. The capacitance of the tuning capacitor 4 for LF1 is set almost equal to the sum of the capacitance of the tuning capacitor 7 for LF1 and the capacitance of the tuning variable capacitor 10 for LF1.
A tuning variable capacitor 12 for LF2 and a matching capacitor 13 for LF2 are connected with the grounded end of one of the inductors 2 and 3. For example, in the example of FIG. 1, the tuning variable capacitor 12 for LF2 and the matching capacitor 13 are connected with the grounded end of the inductor 3. The capacitance of the tuning capacitor 6 for LF2 is set almost equal to the sum of the capacitance of the tuning capacitor 9 and the tuning variable capacitor 12 for LF2.
Under this state, the sample coil 1, inductors 2, 3, tuning capacitors 6, 9, 4, and 7 together form a balanced resonant circuit where the voltage amplitude becomes zero near the center point of the sample coil 1. Each of the inductors 2 and 3 may be a lumped inductor, such as a helical coil fabricated by winding wire like a coil, or may be a distributed inductor, such as a conducting rod. That is, each of the inductors 2 and 3 may be any inductive component element.
The operation of this multiple tuning circuit is next described. FIG. 2 shows an equivalent circuit of the multiple tuning circuit when resonating at LF1. The tuning capacitors 6 and 9 corresponding to the resonant frequency LF2 have large capacitances and show sufficiently low impedances at the frequency of LF1. Therefore, the two inductors 2 and 3 are regarded to be connected in series as shown. Since the tuning capacitor 9 has a large capacitance, the contribution of the tuning variable capacitor 12 and matching variable capacitor 13 on the side of LF2 is small.
The series combination of the two inductors 2 and 3 is connected in parallel with the sample coil 1, thus forming a first resultant inductance. This resultant inductance and the tuning capacitors 4 and 7 corresponding to the resonant frequency LF1 together form a first LC resonant circuit. The tuning variable capacitor 10 and matching variable capacitor 11 on the side of the resonant frequency LF1 together act to tune and match the LC resonant circuit for the resonant frequency LF1. At some instant of time, tuning and matching are made at the resonant frequency LF1, and the resonant current maximizes. One example of the orientation of RF currents under this condition is indicated by the arrows 21 and 22.
An equivalent circuit of the multiple tuning circuit resonating at LF2 is shown in FIG. 3. The tuning capacitors 4 and 7 for the resonant frequency LF1 have small capacitances and show sufficiently large impedances at the frequency of LF2. Therefore, the contribution of the two tuning capacitors 4 and 7 is small. Furthermore, the contribution of the tuning variable capacitor 10 and matching variable capacitor 11 on the side of the resonant frequency LF1 is small, because their capacitances are small.
The inductor 2, sample coil 1, and inductor 3 are connected in turn and in series to thereby form a second resultant inductance. This second resultant inductance and the tuning capacitors 6, 9 for the resonant frequency LF2 together form a second LC resonant circuit. The tuning variable capacitor 12 and matching variable capacitor 13 on the side of the resonant frequency LF2 make tuning and matching at the resonant frequency LF2.
At some instant of time, tuning and matching are made at the resonant frequency LF2, and the resonant current maximizes. One example of the orientation of an RF current under this condition is indicated by the arrow 31.
FIG. 3 indicates an equivalent circuit of the multiple tuning circuit when resonating at the frequency LF2. In this equivalent circuit configuration, the inductor 2, sample coil 1, and inductor 3 are connected in series to form the resultant inductance. Therefore, when an RF current 31 flows, an RF voltage developed across the inductor 2, an RF voltage developed across the sample coil 1, and an RF voltage developed across the inductor 3 are added up. Consequently, a quite high RF voltage relative to ground potential is generated at positions 32 and 33. As a result, electric discharge tends to be produced across each of the positions 32 and 33 and ground. Once such electric discharge occurs, electronic parts, such as capacitors, are burned out. Hence, expensive repair cost is necessary.
In view of the foregoing problems, it is an object of the present invention to provide a multiple tuning circuit which is for use in an NMR spectrometer and has improved voltage resistance and thus is less likely to produce electric discharge even if high RF power is injected.
To achieve this object, the present invention provides a multiple tuning circuit for use in an NMR spectrometer, the tuning circuit comprising: a sample coil having ends A and B, the end A being grounded via a first capacitive element, the end B being grounded via a second capacitive element; a first inductor having one end connected with the end A of the sample coil via a third capacitive element, the other end being grounded; a second inductor having one end connected with the end B of the sample coil via a fourth capacitive element, the other end being grounded; a first set of matching circuit and tuning circuit for supplying first RF to the sample coil; and a second set of matching circuit and tuning circuit for supplying second RF to the sample coil. This multiple tuning circuit is characterized in that the sample coil, first inductor, second inductor, first capacitive element, second capacitive element, third capacitive element, and fourth capacitive element together form a balanced resonant circuit where the amplitude voltage becomes zero near the center point of the sample coil.
In one feature of the multiple tuning circuit, the grounded end of the first inductor is grounded via a fifth capacitive element.
In another feature of the multiple tuning circuit, the grounded end of the second inductor is grounded via a sixth capacitive element.
In a further feature of the multiple tuning circuit, the first set of matching circuit and tuning circuit is directly connected with one end of the sample coil.
In still another feature of the multiple tuning circuit, the second set of matching circuit and tuning circuit is connected between the first inductor and the fifth capacitive element or between the second inductor and the sixth capacitive element.
In yet another feature of the multiple tuning circuit, at least one of the third and fourth capacitive elements is a tuning variable capacitive element for the second RF.
In an additional feature of the multiple tuning circuit, the tuning variable capacitive element for the second RF has one end connected with the junction of the sample coil and the second capacitive element, the other end being connected with the second inductor. The junction of the tuning variable capacitive element for the second RF and the second inductor is grounded via an eighth capacitive element for bringing the first RF into resonance.
In an additional feature of the multiple tuning circuit, the third capacitive element connected with the end A of the sample coil is split into at least two parts connected in series. One end of the first capacitive element and one end of the first set of matching circuit and tuning circuit are connected with the junction of these parts connected in series. The other end of the first capacitive element and the other end of the first set of matching circuit and tuning circuit are grounded. The fourth capacitive element connected with the end B of the sample coil is split into at least two parts connected in series. One end of the second capacitive element for bringing the first RF into resonance is connected with the junction of these parts connected in series. The other end of the second capacitive element is grounded.
In an additional feature of the multiple tuning circuit, a capacitive element of a variable or fixed capacitance is connected in parallel with at least one of the first and second inductors.
The present invention also provides a multiple tuning circuit for use in an NMR spectrometer, the tuning circuit comprising: a sample coil having ends A and B; a first inductor having one end connected with the end A of the sample coil via first and second capacitive elements, the other end being grounded; a second inductor having one end connected with the end B of the sample coil via third and fourth capacitive elements, the other end being grounded; one or more fifth capacitive elements for connecting the junction of the first and second capacitive elements with the junction of the third and fourth capacitive elements directly or indirectly via ground; a first matching circuit for supplying first RF to the sample coil; and a second matching circuit for supplying second RF to the sample coil. This multiple tuning circuit is characterized in that the sample coil, first inductor, second inductor, first capacitive element, second capacitive element, third capacitive element, and fourth capacitive element together form a balanced resonant circuit where the amplitude voltage becomes zero near the center point of the sample coil.
In one feature of this multiple tuning circuit, the grounded end of the first inductor is grounded via a sixth capacitive element.
In another feature of the multiple tuning circuit, the grounded end of the second inductor is grounded via a seventh capacitive element.
In a further feature of this multiple tuning circuit, at least one of the first through fourth capacitive elements is a tuning variable capacitive element for the second RF.
In a still further feature of the multiple tuning circuit, at least one of the fifth capacitive elements is a tuning variable capacitive element for the first RF.
In still another feature of the multiple tuning circuit, a capacitive element having a fixed or variable capacitance is connected in parallel with at least one of the first and second inductors.
In yet another feature of the multiple tuning circuit, the first RF is higher in frequency than the second RF.
Furthermore, the present invention provides a multiple tuning circuit for use in an NMR spectrometer, the tuning circuit comprising: a sample coil having ends A and B; a second inductor having one end connected with the end A of the sample coil via a first capacitive element, a first inductor and a second capacitive element, the other end being grounded; a fourth inductor having one end connected with the end B of the sample coil via a third capacitive element, a third inductor, and a fourth capacitive element; one or more fifth capacitive elements for connecting the junction of the first inductor and the second capacitive element with the junction of the third inductor and the fourth capacitive element directly or indirectly via ground; a first matching circuit for supplying first RF to the sample coil; a second matching circuit for supplying second RF to the sample coil; and a third matching circuit for supplying third RF to the sample coil. This multiple tuning circuit is characterized in that the sample coil, first inductor, second inductor, third inductor, fourth inductor, first capacitive element, second capacitive element, third capacitive element, and fourth capacitive element together form a balanced resonant circuit where the amplitude voltage becomes zero near the center point of the sample coil.
In one feature of this multiple tuning circuit, the grounded end of the second inductor is grounded via a sixth capacitive element.
In another feature of this multiple tuning circuit, the grounded end of the fourth inductor is grounded via a seventh capacitive element.
In still another feature of this multiple tuning circuit, at least one of the first through fourth capacitive elements is a tuning variable capacitive element for the second RF.
In yet another feature of this multiple tuning circuit, at least one capacitive element of the fifth capacitive elements is a tuning variable capacitive element for the first RF.
In an additional feature of this multiple tuning circuit, a tuning capacitive element for the third RF is formed at least in a given position in the first inductor or in a given position in the third inductor.
In an additional feature of this multiple tuning circuit, a capacitive element having a variable or fixed capacitance is connected in parallel with at least one of the second and fourth inductors.
In an additional feature of this multiple tuning circuit, a band-reject filter for rejecting the second RF is inserted between an input terminal for the first RF and a matching capacitive element.
In an additional feature of this multiple tuning circuit, a band-reject filter for rejecting the first RF is inserted between an input terminal for the second RF and a matching capacitive element.
In an additional feature of this multiple tuning circuit, the third RF is higher in frequency than the first RF. The first RF is higher in frequency than the second RF.
In addition, the present invention provides a probe for use in an NMR spectrometer, said probe including a multiple tuning circuit which comprises: a sample coil having ends A and B, the end A being grounded via a first capacitive element, the end B being grounded via a second capacitive element; a first inductor having one end connected with the end A of the sample coil via a third capacitive element, the other end being grounded; a second inductor having one end connected with the end B of the sample coil via a fourth capacitive element, the other end being grounded; a first set of matching circuit and tuning circuit for supplying first RF to the sample coil; and a second set of matching circuit and tuning circuit for supplying second RF to the sample coil. The multiple tuning circuit has an electrical circuit portion whose outside is surrounded by a cylindrical electrode that is used as a grounding electrode of the multiple tuning circuit.
Moreover, the present invention provides a probe for use in an NMR spectrometer, the probe including a multiple tuning circuit which comprises: a sample coil having ends A and B; a first inductor having one end connected with the end A of the sample coil via first and second capacitive elements, the other end being grounded; a second inductor having one end connected with the end B of the sample coil via third and fourth capacitive elements, the other end being grounded; one or more fifth capacitive elements for connecting the junction of the first capacitive element and the second capacitive element with the junction of the third and fourth capacitive elements directly or indirectly via ground; a first matching circuit for supplying first RF to the sample coil; and a second matching circuit for supplying second RF to the sample coil. The multiple tuning circuit has an electrical circuit portion whose outside is surrounded by a cylindrical electrode that is used as a grounding electrode of the multiple tuning circuit.
Additionally, the present invention provides a probe for use in an NMR spectrometer, the probe including a multiple tuning circuit which comprises: a sample coil having ends A and B; a second inductor having one end connected with the end A of the sample coil via a first capacitive element, a first inductor, and a second capacitive element, the other end being grounded; a fourth inductor having one end connected with the end B of the sample coil via a third capacitive element, a third inductor, and a fourth capacitive element, the other end being grounded; one or more fifth capacitive elements for connecting the junction of the first inductor and the second capacitive element with the junction of the third inductor and the fourth capacitive element directly or indirectly via ground; a first matching circuit for supplying first RF to the sample coil; a second matching circuit for supplying second RF to the sample coil; and a third matching circuit for supplying third RF to the sample coil. The multiple tuning circuit has an electrical circuit portion surrounded by a cylindrical electrode that is used as a grounding electrode for the multiple tuning circuit.
In one feature of this probe, the cylindrical electrode has a window in a given position to place the inside of the probe in communication with the outside.
Further, the present invention provides a probe for use in an NMR spectrometer, the probe comprising: a sample coil having ends A and B; a first capacitor having one end connected with the end A of the sample coil, the other end being grounded; and a second capacitor having one end connected with the end B of the sample coil, the other end being grounded. The sample coil, first capacitor, and second capacitor together form a balanced resonant circuit for bringing RF amplitude voltage near the center point of the sample coil to zero. The multiple tuning circuit has an electrical circuit portion whose outside is surrounded by a cylindrical electrode that is used as a grounding electrode of the electrical circuit portion.
In one feature of this probe, the cylindrical electrode has a window in a given position to place the inside of the probe in communication with the outside.
Other objects and features of the present invention will appear in the course of the description thereof, which follows.